Biaxially oriented film laminates are used commercially on a large scale in shrink packaging of food items. For example, biaxially oriented polyvinylidene chloride (PVDC) laminates have been used extensively in manufacture of packaged meat and cheese products, in part because these laminates exhibit excellent shrink characteristics. Although the physical properties of PVDC are well suited to packaging applications, a number of processability deficiencies are associated with its use. In particular, PVDC is thermally unstable, necessitating the use of specialized equipment to handle PVDC film. Moreover, PVDC is prone to development of gels and carbonized particles on exposure to high temperature for extended periods. Therefore, special care must be taken during PVDC film manufacture in order to produce high quality product having minimal levels of black specks. PVDC is also known to discolor during electron beam irradiation. In addition, shrinkbags that are formed from multilayer films of PVDC and ethylene vinyl acetate (EVA) are deficient in toughness. To address this problem, patches are often placed on the bags to provide additional protection for the packaged products, for example in packaging of bone-in meat. The patches are unsightly and placement requires additional processing steps.
A number of nylon-based compositions adapted for use in shrinkbags have been developed as a means to overcome the various shortcomings of PVDC bags. For example, U.S. Pat. No. 5,053,259 describes an oriented multilayer film comprising a blend of amorphous nylon copolymer and a copolyamide having a melting point of at least 145° C. The first outer layer comprises a polyolefin, ionomer, PVDC, EVA or ethylene vinyl alcohol (EVOH) copolymer or mixtures thereof. The compositions of the examples exhibit shrinkage values at 90° C. of 15% to 51%. European Patent 0987103B1 describes a multilayer film that is biaxially oriented at a stretch ratio of greater or equal to 3:1 in the machine direction and 6:1 in the transverse direction. The film has an outer heat sealing layer comprising a polyolefin, including copolymers of ethylene and polar comonomers and another layer of crystalline or partially crystalline copolyamide, optionally containing EVOH. The film has a free shrink in each direction of at least 10% at 120° C. and compositions are disclosed that exhibit free shrink of up to 56% at 120° C.
Although shrinkbags that are composed of nylon have desirable properties, they also exhibit certain property limitations. For example, such bags tend to be stiff compared to PVDC-based shrinkbags and may be less compatible with many packaging operations that are suitable for the softer PVDC bags. Also, nylon shrinkbags cannot achieve shrink levels after orientation that are as high as those of PVDC shrinkbags. This is due to post-crystallization of the nylon-containing film that occurs at room temperature after the extrusion and orientation processes are complete. In consequence, variable and unpredictable degrees of shrinkage occur depending on storage conditions. Post-crystallization occurs to the greatest extent within the first two to three weeks after manufacture of the oriented film. It is common for commercially produced PVDC bags to attain 50 to 60% shrink at 95° C., both off-line and after storage or aging at room temperature for several weeks. In contrast, nylon-containing shrinkbags rarely exhibit greater than 40% shrink at 95° C. after storage or aging. In general, shrinkages of between 20% and 40% after aging are typical for nylon-containing films. Shrinkage at temperatures below 95° C. is even less. Bag manufacturers are forced to compensate for this deficiency by manufacturing lay-flat bags in a larger size to accommodate this phenomenon. In addition, this necessitates maintaining inventories of various sized bags, which is inefficient.
Shrink films and shrinkbags having barrier layers comprising EVOH are also known. For example, European Patent 0217252B1 describes a film comprising an outer polymer layer and an inner heat sealing layer, optionally having an EVOH oxygen barrier layer between the inner and outer layers. The heat-sealing layer comprises a linear copolymer of ethylene and an α-olefin. The heat sealing layer may further comprise of a polymer which is compatible with the ethylene/α-olefin copolymer and is selected from the group consisting of linear low density polyethylene, linear high density polyethylene, low density polyethylene, EVA, acid-modified EVA, polypropylene, ethylene/propylene copolymers, ionomers and ethylene/alkyl acrylate copolymers. The polymer of the outer layer may be selected from copolymers of ethylene and an α-olefin, EVA, polyamide, and ionomeric resins and the barrier layer can be PVDC or EVOH.
European Patent 0447988B1 discloses a heat-shrinkable multilayer thermoplastic film having (a) a thermosealing layer, (b) a gas barrier layer selected from copolymers of vinylidene chloride, EVOH copolymers, polyamides or copolyamides and blends of these polymers; (c) at least one polymeric structural layer formed of a polymer selected from ethylene copolymers of very low density polyethylene, high density polyethylene, linear low density polyethylene, ethylene copolymers with vinyl or acrylic comonomers, ionomers, olefinic polymers and copolymers modified with functional groups, polyamides, polyesters, copolyesters, and blends of these polymers, and (d) optionally at least one adhesive layer.
Co-pending U.S. patent application Ser. No. 11/644,976 discloses a heat shrinkable multilayer film wherein a first layer comprises a polymer selected from the group consisting of EVOH, polyamides and mixtures thereof, a second layer comprises a polymer selected from the group consisting of polyethylene homopolymers, ethylene copolymers, polypropylene homopolymers, propylene copolymers, polyesters, polyamides, polyvinyl chloride, polycarbonates, cyclic olefin polymers and mixtures thereof and a third adhesive layer is present, positioned between and in contact with the first and second layers. The third layer consists essentially of a blend of a two ethylene copolymers, one of which is a functionalized ethylene copolymer, and a tackifier resin.
European Patent 0571260 discloses a film comprising two exterior layers, the layers composed of at least a thermoplastic ionomer resin; an internal barrier layer comprising a thermoplastic ethylene vinyl alcohol copolymer with a semi-crystalline structure; and preferably adhesive layers situated between the barrier layer and each of the outer layers. The adhesive layers are preferably ethylene vinyl acetate copolymers.
European Patent 1563990 discloses a multi-layer, biaxially oriented and biaxially heat-shrinkable, gas barrier, thermoplastic film composed of a first outer layer comprising ethylene homo- or copolymers; a core gas barrier layer such as EVOH; a second outer thermoplastic layer; and an inner layer comprising terpolymers of propylene, ethylene and C4-C8 alpha-olefins.
U.S. Pat. No. 4,939,040 discloses a stretchable multilayer film with gas permeability below 50 cm3 O2/24 hr/m2/atm., comprising a barrier layer prepared from, for example, EVOH containing 1 to 20 weight % of a stiffness modifier; and stretchable layer(s) preferably selected from copolymers of ethylene with vinyl acetate, methyl acrylate or butyl acrylate; optionally further comprising at least one outer thermosealing layer prepared from, among others, ionomeric resins or EVA.
U.S. Pat. No. 6,146,726 provides a heat-shrinkable multilayer film comprising an outermost layer, a gas barrier layer as a core layer and a sealing layer as the innermost layer, where the sealing layer is formed of a material comprising a linear ethylene-1-octene copolymer obtained by using a constrained geometry catalyst. An intermediate layer between the outermost layer and the core layer is formed of at least one resin selected from the group consisting of polyamide resins, thermoplastic polyester resins and ethylene copolymer resins. The gas barrier layer can be EVOH. The outermost layer is formed of at least one polyethylene terephthalate copolyester resin.
U.S. Pat. No. 5,387,470 discloses a multilayer film comprising a first core layer comprising very low density polyethylene or linear low density polyethylene and two outer layers each comprising an ionomer. The film may also include additional core layers and/or an oxygen barrier layer such as EVOH and may be oriented to provide a heat shrinkable film.
EVOH film laminates of these types have excellent barrier properties. However, EVOH can be challenging to orient in multilayer structures. The difficulty in stretching EVOH has been attributed in part to a particular mechanical characteristic of EVOH. That is, with increasing orientation temperature, stress-induced crystallization rates increase dramatically. As a result, higher levels of crystallinity are found in EVOH that is oriented at temperatures of about 70° C. to about 160° C. compared to EVOH that is heated, but not oriented at these temperatures. (See R. Chou & I. H. Lee, The Behavior of Ethylene Vinyl Alcohol Amorphous Nylon Blends in Solid Phase Thermoforming, J. Plastic Film & Sheeting, 13, 74-93, 1997). Rupture of the existing crystal morphology takes place in EVOH laminated films that have been oriented at these temperatures, leading to necking of the EVOH layer and creation of breaks in the EVOH, thereby making it difficult to obtain uniform stretching of the laminated film. Another aspect related to stretching of EVOH is the presence of a sharp yield point, as measured in stress-strain curves, followed by strain-hardening. These deformation phenomena lead to propagation of necking during orientation. It has been shown that when drawing takes place at 80° C., deformation is homogeneous, but becomes much more heterogeneous at drawing temperatures of 120° C. (See K. Djezzar et al., “Tensile Drawing of Ethylene/Vinyl Alcohol Polymers: Part 1, Influence of Draw Temperature on the Mechanical Behaviour”, Polymer, 39, 3945, 1998). To further compound the difficulty of orienting EVOH, when EVOH is coextruded with other polymers in a multilayer structure the softening temperature range of the polymers in the other layers of the multilayer film frequently does not overlap well with the optimal stretching temperature range for EVOH, resulting in stretching of the combined structure that is not optimal (see T. Okaya & K. Ikari, Chapter 8, Ethylene Vinyl Alcohol Copolymers, Polyvinyl Alcohol Developments, (C. A. Finch, ed.) John Wiley, New York, 1992; and R. Chou & I. H. Lee, supra). Eval Americas Technical Bulletin No. 150 “Thermoforming of Eval® Resin Containing Structures” describes the art of thermoforming as being very similar to the process of biaxially orientating polyolefins. As disclosed therein, during solid phase pressure forming of polypropylene EVOH sheet, the use of 32 mole % ethylene EVOH, such as EVAL F101 ethylene vinyl alcohol copolymer resin is not recommended, as the rapid crystallization rate of EVOH at the optimum forming temperature range for polypropylene leads to the formation of cracks.
Various approaches to improving the stretchability of EVOH have been developed. Most of these approaches have involved blending of other polymers into EVOH. For example, U.S. Pat. No. 5,126,402 describes a blend of ethylene vinyl alcohol copolymer blended with 5 to 95 weight % of an amorphous polyamide having fewer than about 0.100 equivalents of carboxyl end groups per kilogram of polyamide. Such a blend provides improved thermal and oxidative stability while providing desirable formability. U.S. Pat. No. 4,911,979 describes a heat shrinkable laminated film comprising at least two layers of a mixed resin layer consisting essentially of 65 to 85 weight % polyamide resin and 15 to 35 weight % of a saponified ethylene vinyl acetate; and a layer containing at least 55 weight % of a saponified ethylene vinyl acetate. Such a laminate can be biaxially stretched while heating the laminate to a temperature of 75° C. but not higher than 100° C. U.S. Pat. No. 5,286,575 describes a blend of ethylene vinyl alcohol copolymer blended with 5 to 95 weight % of an amorphous polyamide copolymer having a glass transition temperature of about 30 to 100° C. with improved simultaneous orientation properties. U.S. Pat. No. 5,037,703 describes a multilayer structure where the ethylene vinyl alcohol copolymer is blended with 5 to 50 weight % of a thermoplastic polyester comprising at least 50 mol % of isophthalic acid based on the total moles of carboxylic acid component and 0.1 to 30 mol % of 1,3 bis(beta-hydroxyethoxy)-benzene and/or diethylene glycol, based on the total moles of diol component. Such a multilayer structure is said to prevent formation of pinholes or cracks during high-speed heat stretching.
Other approaches to improving drawability and stretchability include the use of modified EVOH copolymers and mixtures of EVOH polymers having different properties. For example, U.S. Pat. No. 4,713,296 describes a laminate of modified ethylene vinyl alcohol copolymer containing 0.1 to 5 mol % pyrrolidone ring containing units where the laminate is superior in drawability. U.S. Pat. No. 6,372,359B1 describes a multilayer film of polypropylene with a resin composition comprising two ethylene vinyl alcohol copolymers with different and partially overlapping melt temperature ranges. Such a composition provides high stretchability and excellent gas barrier properties.
It would be desirable to have available multilayer biaxially orientable shrink films of enhanced properties that incorporate EVOH but that also have excellent outer layer toughness and the stiffness exhibited by PVDC. Further, it would be useful if the film laminates were characterized by high shrink with minimal loss of shrinkage over time and could be produced using conventional shrink film production processes and temperatures. If such films additionally exhibited high shrink percentages at temperatures at or below 95° C. they could be used advantageously to produce shrinkbags that would enable more effective preservation of food products.